Hypertext Transfer Protocol
Encyclopedia
The Hypertext Transfer Protocol (HTTP) is a networking protocol
for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web
.
The standards development of HTTP has been coordinated by the Internet Engineering Task Force
(IETF) and the World Wide Web Consortium
(W3C), culminating in the publication of a series of Requests for Comments (RFCs), most notably RFC 2616 (June 1999), which defines HTTP/1.1, the version of HTTP in common use.
protocol in the client-server
computing model. In HTTP, a web browser
, for example, acts as a client, while an application running on a computer hosting
a web site functions as a server. The client submits an HTTP request message to the server. The server, which stores content, or provides resources, such as HTML
files, or performs other functions on behalf of the client, returns a response message to the client. A response contains completion status information about the request and may contain any content requested by the client in its message body.
A web browser (or client) is often referred to as a user agent
(UA). Other user agents can include the indexing software used by search providers, known as web crawler
s, or variations of the web browser such as voice browser
s, which present an interactive voice user interface.
The HTTP protocol is designed to permit intermediate network elements to improve or enable communications between clients and servers. High-traffic websites often benefit from web cache
servers that deliver content on behalf of the original, so-called origin server, to improve response time. HTTP proxy server
s at network boundaries facilitate communication when clients without a globally routable address are located in private network
s by relaying the requests and responses between clients and servers.
HTTP is an Application Layer
protocol designed within the framework of the Internet Protocol Suite
. The protocol definitions presume a reliable Transport Layer
protocol for host-to-host data transfer. The Transmission Control Protocol
(TCP) is the dominant protocol in use for this purpose. However, HTTP has found application even with unreliable protocols, such as the User Datagram Protocol
(UDP) in methods such as the Simple Service Discovery Protocol
(SSDP).
HTTP Resources
are identified and located on the network by Uniform Resource Identifier
s (URIs)—or, more specifically, Uniform Resource Locator
s (URLs)—using the http or https
URI scheme
s. URIs and the Hypertext Markup Language (HTML), form a system of inter-linked resources, called hypertext
documents, on the Internet
, that led to the establishment of the World Wide Web
in 1990 by English computer scientist and innovator Tim Berners-Lee
.
The original version of HTTP (HTTP/1.0) was revised in HTTP/1.1. HTTP/1.0 uses a separate connection to the same server for every request-response transaction, while HTTP/1.1 can reuse a connection multiple times, to download, for instance, images for a just delivered page. Hence HTTP/1.1 communications experience less latency
as the establishment of TCP connections presents considerable overhead.
was coined by Ted Nelson
who in turn was inspired by Vannevar Bush
's microfilm-based "memex
". Tim Berners-Lee
first proposed the "WorldWideWeb" project — now known as the World Wide Web
. Berners-Lee and his team are credited with inventing the original HTTP protocol along with the HTML and the associated technology for a web server and a text-based web browser.
The first version of the protocol had only one method
, namely GET, which would request a page from a server. The response from the server was always an HTML page.
The first documented version of HTTP was HTTP V0.9 (1991). Dave Raggett
led the HTTP Working Group (HTTP WG) in 1995 and wanted to expand the protocol extended operations, extended negotiation, richer meta-information, tied with a security protocol and got more efficient by adding additional methods and header fields. RFC 1945 officially introduced and recognized HTTP V1.0 in 1996.
The HTTP WG planned to publish new standards in December 1995 and the support for pre-standard HTTP/1.1 based on the then developing RFC 2068 (called HTTP-NG) was rapidly adopted by the major browser developers in early 1996. By March 1996, pre-standard HTTP/1.1 was supported in Arena
, Netscape 2.0
, Netscape Navigator Gold 2.01, Mosaic 2.7
, Lynx 2.5
, and in Internet Explorer 3.0
. End user adoption of the new browsers was rapid. In March 1996, one web hosting company reported that over 40% of browsers in use on the Internet were HTTP 1.1 compliant. That same web hosting company reported that by June 1996, 65% of all browsers accessing their servers were HTTP/1.1 compliant. The HTTP/1.1 standard as defined in RFC 2068 was officially released in January 1997. Improvements and updates to the HTTP/1.1 standard were released under RFC 2616 in June 1999.
(TCP) connection to a particular port
on a server (typically port 80; see List of TCP and UDP port numbers). An HTTP server listening on that port waits for a client's request message. Upon receiving the request, the server sends back a status line, such as "HTTP/1.1 200 OK", and a message of its own, the body of which is perhaps the requested resource, an error message, or some other information.
The request line and headers must all end with (that is, a carriage return
followed by a line feed). The empty line must consist of only and no other whitespace
. Although is required alone is also accepted by most servers. In the HTTP/1.1 protocol, all headers except Host are optional.
A request line containing only the path name is accepted by servers to maintain compatibility with HTTP clients before the HTTP/1.0 specification in RFC1945.
HTTP defines nine methods (sometimes referred to as "verbs") indicating the desired action to be performed on the identified resource. What this resource represents, whether pre-existing data or data that is generated dynamically, depends on the implementation of the server. Often, the resource corresponds to a file or the output of an executable residing on the server.
HEAD: Asks for the response identical to the one that would correspond to a GET request, but without the response body. This is useful for retrieving meta-information written in response headers, without having to transport the entire content.
GET: Requests a representation of the specified resource. Requests using GET (and a few other HTTP methods) "SHOULD NOT have the significance of taking an action other than retrieval
". The W3C has published guidance principles on this distinction, saying, "Web application
design should be informed by the above principles, but also by the relevant limitations." See safe methods below.
POST
: Submits data to be processed (e.g., from an HTML form) to the identified resource. The data is included in the body of the request. This may result in the creation of a new resource or the updates of existing resources or both.
PUT: Uploads a representation of the specified resource.
DELETE: Deletes the specified resource.
TRACE: Echoes back the received request, so that a client can see what (if any) changes or additions have been made by intermediate servers.
OPTIONS: Returns the HTTP methods that the server supports for specified URL. This can be used to check the functionality of a web server by requesting '*' instead of a specific resource.
CONNECT: Converts the request connection to a transparent TCP/IP tunnel
, usually to facilitate SSL
-encrypted communication (HTTPS
) through an unencrypted HTTP proxy.
PATCH: Is used to apply partial modifications to a resource.
HTTP servers are required to implement at least the GET and HEAD methods and, whenever possible, also the OPTIONS method.
, beyond relatively harmless effects such as logging
, caching
, the serving of banner advertisements
or incrementing a web counter
. Making arbitrary GET requests without regard to the context of the application's state should therefore be considered safe.
By contrast, methods such as POST, PUT and DELETE are intended for actions that may cause side effects either on the server, or external side effects such as financial transactions or transmission of email
. Such methods are therefore not usually used by conforming web robots
or web crawler
s; some that do not conform tend to make requests without regard to context or consequences.
Despite the prescribed safety of GET requests, in practice their handling by the server is not technically limited in any way. Therefore, careless or deliberate programming can cause non-trivial changes on the server. This is discouraged, because it can cause problems for Web caching, search engines and other automated agents, which can make unintended changes on the server.
Furthermore, methods such as TRACE, TRACK and DEBUG are considered potentially 'unsafe' by some security professionals, because they can be used by attackers to gather information or bypass security controls during attacks. Security software tools such as Tenable Nessus and Microsoft URLScan report on the presence of these methods as being security issues.
In contrast, the POST method is not necessarily idempotent, and therefore sending an identical POST request multiple times may further affect state or cause further side effects (such as financial transactions). In some cases this may be desirable, but in other cases this could be due to an accident, such as when a user does not realize that their action will result in sending another request, or they did not receive adequate feedback that their first request was successful. While web browser
s may show alert dialog box
es to warn users in some cases where reloading a page may re-submit a POST request, it is generally up to the web application to handle cases where a POST request should not be submitted more than once.
Note that whether a method is idempotent is not enforced by the protocol or web server. It is perfectly possible to write a web application in which (for example) a database insert or other non-idempotent action is triggered by a GET or other request. Ignoring this recommendation, however, may result in undesirable consequences, if a user agent
assumes that repeating the same request is safe when it isn't.
") and a textual reason phrase (such as "Not Found"). The way the user agent
handles the response primarily depends on the code and secondarily on the response headers. Custom status codes can be used since, if the user agent encounters a code it does not recognize, it can use the first digit of the code to determine the general class of the response.
Also, the standard reason phrases are only recommendations and can be replaced with "local equivalents" at the web developer
's discretion. If the status code indicated a problem, the user agent might display the reason phrase to the user to provide further information about the nature of the problem. The standard also allows the user agent to attempt to interpret the reason phrase, though this might be unwise since the standard explicitly specifies that status codes are machine-readable and reason phrases are human-readable.
Such persistent connections reduce request latency
perceptibly, because the client does not need to re-negotiate the TCP connection after the first request has been sent.
Version 1.1 of the protocol made bandwidth optimization improvements to HTTP/1.0. For example, HTTP/1.1 introduced chunked transfer encoding
to allow content on persistent connections to be streamed, rather than buffered. HTTP pipelining
further reduces lag time, allowing clients to send multiple requests before a previous response has been received to the first one. Another improvement to the protocol was byte serving
, which is when a server transmits just the portion of a resource explicitly requested by a client.
for a user, the web application
may have to track the user's progress from page to page. A common solution is the use of HTTP cookie
s. Other methods include server side sessions, hidden variables (when the current page is a form
), and URL-rewriting using URI-encoded parameters, e.g., /index.php?session_id=some_unique_session_code.
and the HTTP/1.1 Upgrade header
. Browser support for the latter two is, however, nearly non-existent, so HTTP Secure is the dominant method of establishing a secure HTTP connection.
, port 80.
A client request (consisting in this case of the request line and only one header) is followed by a blank line, so that the request ends with a double newline, each in the form of a carriage return
followed by a line feed. The "Host" header distinguishes between various DNS
names sharing a single IP address
, allowing name-based virtual hosting
. While optional in HTTP/1.0, it is mandatory in HTTP/1.1.
A server response is followed by a blank line and text of the requested page. The ETag
(entity tag) header is used to determine if a cached version of the requested resource is identical to the current version of the resource on the server. Content-Type specifies the Internet media type
of the data conveyed by the http message, while Content-Length indicates its length in bytes. The HTTP/1.1 webserver publishes its ability to respond to requests for certain byte ranges of the document by setting the header Accept-Ranges: bytes. This is useful, if the client needs to have only certain portions of a resource sent by the server, which is called byte serving
. When Connection: close is sent in a header, it means that the web server
will close the TCP
connection immediately after the transfer of this response.
Most of the header lines are optional. When Content-Length is missing the length is determined in other ways. Chunked transfer encoding uses a chunk size of 0 to mark the end of the content. Identity encoding without Content-Length reads content until the socket is closed.
A Content-Encoding like gzip can be used to compress the transmitted data.
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
.
The standards development of HTTP has been coordinated by the Internet Engineering Task Force
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
(IETF) and the World Wide Web Consortium
World Wide Web Consortium
The World Wide Web Consortium is the main international standards organization for the World Wide Web .Founded and headed by Tim Berners-Lee, the consortium is made up of member organizations which maintain full-time staff for the purpose of working together in the development of standards for the...
(W3C), culminating in the publication of a series of Requests for Comments (RFCs), most notably RFC 2616 (June 1999), which defines HTTP/1.1, the version of HTTP in common use.
Technical overview
HTTP functions as a request-responseRequest-response
Request-response or request-reply is one of the basic methods computers use to talk to each other. When using request-response, the first computer requests some data and the second computer responds to the request. Usually there is a series of such interchanges until the complete message is sent. ...
protocol in the client-server
Client-server
The client–server model of computing is a distributed application that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients. Often clients and servers communicate over a computer network on separate hardware, but both...
computing model. In HTTP, a web browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
, for example, acts as a client, while an application running on a computer hosting
Host (network)
A network host is a computer connected to a computer network. A network host may offer information resources, services, and applications to users or other nodes on the network. A network host is a network node that is assigned a network layer host address....
a web site functions as a server. The client submits an HTTP request message to the server. The server, which stores content, or provides resources, such as HTML
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
files, or performs other functions on behalf of the client, returns a response message to the client. A response contains completion status information about the request and may contain any content requested by the client in its message body.
A web browser (or client) is often referred to as a user agent
User agent
In computing, a user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system...
(UA). Other user agents can include the indexing software used by search providers, known as web crawler
Web crawler
A Web crawler is a computer program that browses the World Wide Web in a methodical, automated manner or in an orderly fashion. Other terms for Web crawlers are ants, automatic indexers, bots, Web spiders, Web robots, or—especially in the FOAF community—Web scutters.This process is called Web...
s, or variations of the web browser such as voice browser
Voice browser
A voice browser is a web browser that presents an interactive voice user interface to the user. In addition, it typically provides an interface to the PSTN or a PBX. Just as a visual web browser works with HTML pages, a voice browser operates on pages that specify voice dialogues...
s, which present an interactive voice user interface.
The HTTP protocol is designed to permit intermediate network elements to improve or enable communications between clients and servers. High-traffic websites often benefit from web cache
Web cache
A web cache is a mechanism for the temporary storage of web documents, such as HTML pages and images, to reduce bandwidth usage, server load, and perceived lag...
servers that deliver content on behalf of the original, so-called origin server, to improve response time. HTTP proxy server
Proxy server
In computer networks, a proxy server is a server that acts as an intermediary for requests from clients seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource available from a different server...
s at network boundaries facilitate communication when clients without a globally routable address are located in private network
Private network
In the Internet addressing architecture, a private network is a network that uses private IP address space, following the standards set by RFC 1918 and RFC 4193. These addresses are commonly used for home, office, and enterprise local area networks , when globally routable addresses are not...
s by relaying the requests and responses between clients and servers.
HTTP is an Application Layer
Application layer
The Internet protocol suite and the Open Systems Interconnection model of computer networking each specify a group of protocols and methods identified by the name application layer....
protocol designed within the framework of the Internet Protocol Suite
Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
. The protocol definitions presume a reliable Transport Layer
Transport layer
In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
protocol for host-to-host data transfer. The Transmission Control Protocol
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP) is the dominant protocol in use for this purpose. However, HTTP has found application even with unreliable protocols, such as the User Datagram Protocol
User Datagram Protocol
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
(UDP) in methods such as the Simple Service Discovery Protocol
Simple Service Discovery Protocol
The Simple Service Discovery Protocol is a network protocol based on the Internet Protocol Suite for advertisement and discovery of network services and presence information...
(SSDP).
HTTP Resources
Resource (Web)
The concept of resource is primitive in the Web architecture, and is used in the definition of its fundamental elements. The term was first introduced to refer to targets of Uniform Resource Locators , but its definition has been further extended to include the referent of any Uniform Resource...
are identified and located on the network by Uniform Resource Identifier
Uniform Resource Identifier
In computing, a uniform resource identifier is a string of characters used to identify a name or a resource on the Internet. Such identification enables interaction with representations of the resource over a network using specific protocols...
s (URIs)—or, more specifically, Uniform Resource Locator
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
s (URLs)—using the http or https
Https
Hypertext Transfer Protocol Secure is a combination of the Hypertext Transfer Protocol with SSL/TLS protocol to provide encrypted communication and secure identification of a network web server...
URI scheme
URI scheme
In the field of computer networking, a URI scheme is the top level of the Uniform Resource Identifier naming structure. All URIs and absolute URI references are formed with a scheme name, followed by a colon character , and the remainder of the URI called the scheme-specific part...
s. URIs and the Hypertext Markup Language (HTML), form a system of inter-linked resources, called hypertext
Hypertext
Hypertext is text displayed on a computer or other electronic device with references to other text that the reader can immediately access, usually by a mouse click or keypress sequence. Apart from running text, hypertext may contain tables, images and other presentational devices. Hypertext is the...
documents, on the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, that led to the establishment of the World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
in 1990 by English computer scientist and innovator Tim Berners-Lee
Tim Berners-Lee
Sir Timothy John "Tim" Berners-Lee, , also known as "TimBL", is a British computer scientist, MIT professor and the inventor of the World Wide Web...
.
The original version of HTTP (HTTP/1.0) was revised in HTTP/1.1. HTTP/1.0 uses a separate connection to the same server for every request-response transaction, while HTTP/1.1 can reuse a connection multiple times, to download, for instance, images for a just delivered page. Hence HTTP/1.1 communications experience less latency
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...
as the establishment of TCP connections presents considerable overhead.
History
The term HyperTextHypertext
Hypertext is text displayed on a computer or other electronic device with references to other text that the reader can immediately access, usually by a mouse click or keypress sequence. Apart from running text, hypertext may contain tables, images and other presentational devices. Hypertext is the...
was coined by Ted Nelson
Ted Nelson
Theodor Holm Nelson is an American sociologist, philosopher, and pioneer of information technology. He coined the terms "hypertext" and "hypermedia" in 1963 and published it in 1965...
who in turn was inspired by Vannevar Bush
Vannevar Bush
Vannevar Bush was an American engineer and science administrator known for his work on analog computing, his political role in the development of the atomic bomb as a primary organizer of the Manhattan Project, the founding of Raytheon, and the idea of the memex, an adjustable microfilm viewer...
's microfilm-based "memex
Memex
The memex is the name given by Vannevar Bush to the hypothetical proto-hypertext system he described in his 1945 The Atlantic Monthly article As We May Think...
". Tim Berners-Lee
Tim Berners-Lee
Sir Timothy John "Tim" Berners-Lee, , also known as "TimBL", is a British computer scientist, MIT professor and the inventor of the World Wide Web...
first proposed the "WorldWideWeb" project — now known as the World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
. Berners-Lee and his team are credited with inventing the original HTTP protocol along with the HTML and the associated technology for a web server and a text-based web browser.
The first version of the protocol had only one method
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
, namely GET, which would request a page from a server. The response from the server was always an HTML page.
The first documented version of HTTP was HTTP V0.9 (1991). Dave Raggett
Dave Raggett
Dave Raggett is a computer specialist who has played a major role in implementing the World Wide Web since 1992.He has been a W3C Fellow at the World Wide Web Consortium since 1995 and worked on many of the key web protocols, including HTTP, HTML, XHTML, MathML, XForms, and VoiceXML.Raggett wrote...
led the HTTP Working Group (HTTP WG) in 1995 and wanted to expand the protocol extended operations, extended negotiation, richer meta-information, tied with a security protocol and got more efficient by adding additional methods and header fields. RFC 1945 officially introduced and recognized HTTP V1.0 in 1996.
The HTTP WG planned to publish new standards in December 1995 and the support for pre-standard HTTP/1.1 based on the then developing RFC 2068 (called HTTP-NG) was rapidly adopted by the major browser developers in early 1996. By March 1996, pre-standard HTTP/1.1 was supported in Arena
Arena (web browser)
The Arena browser was an early testbed web browser and web authoring tool for Unix. Originally authored by Dave Raggett in 1993, the browser continued its development at CERN and the World Wide Web Consortium and subsequently by Yggdrasil Computing...
, Netscape 2.0
Netscape Navigator
Netscape Navigator was a proprietary web browser that was popular in the 1990s. It was the flagship product of the Netscape Communications Corporation and the dominant web browser in terms of usage share, although by 2002 its usage had almost disappeared...
, Netscape Navigator Gold 2.01, Mosaic 2.7
Mosaic (web browser)
Mosaic is the web browser credited with popularizing the World Wide Web. It was also a client for earlier protocols such as FTP, NNTP, and gopher. Its clean, easily understood user interface, reliability, Windows port and simple installation all contributed to making it the application that opened...
, Lynx 2.5
Lynx (web browser)
Lynx is a text-based web browser for use on cursor-addressable character cell terminals and is very configurable.-Usage:Browsing in Lynx consists of highlighting the chosen link using cursor keys, or having all links on a page numbered and entering the chosen link's number. Current versions support...
, and in Internet Explorer 3.0
Internet Explorer 3
Microsoft Internet Explorer 3 is a graphical web browser released on August 13, 1996 by Microsoft for Microsoft Windows and on January 8, 1997 for Apple Mac OS . It began serious competition against Netscape Navigator in the first Browser war...
. End user adoption of the new browsers was rapid. In March 1996, one web hosting company reported that over 40% of browsers in use on the Internet were HTTP 1.1 compliant. That same web hosting company reported that by June 1996, 65% of all browsers accessing their servers were HTTP/1.1 compliant. The HTTP/1.1 standard as defined in RFC 2068 was officially released in January 1997. Improvements and updates to the HTTP/1.1 standard were released under RFC 2616 in June 1999.
HTTP session
An HTTP session is a sequence of network request-response transactions. An HTTP client initiates a request by establishing a Transmission Control ProtocolTransmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP) connection to a particular port
TCP and UDP port
In computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint in a computer's host operating system. A port is associated with an IP address of the host, as well as the type of protocol used for communication...
on a server (typically port 80; see List of TCP and UDP port numbers). An HTTP server listening on that port waits for a client's request message. Upon receiving the request, the server sends back a status line, such as "HTTP/1.1 200 OK", and a message of its own, the body of which is perhaps the requested resource, an error message, or some other information.
Request message
The request message consists of the following:- Request line, such as GET /images/logo.png HTTP/1.1, which requests a resource called /images/logo.png from server
- Headers, such as Accept-Language: en
- An empty line.
- An optional message body.
The request line and headers must all end with
Carriage return
Carriage return, often shortened to return, refers to a control character or mechanism used to start a new line of text.Originally, the term "carriage return" referred to a mechanism or lever on a typewriter...
followed by a line feed). The empty line must consist of only
Whitespace (computer science)
In computer science, whitespace is any single character or series of characters that represents horizontal or vertical space in typography. When rendered, a whitespace character does not correspond to a visual mark, but typically does occupy an area on a page...
. Although
A request line containing only the path name is accepted by servers to maintain compatibility with HTTP clients before the HTTP/1.0 specification in RFC1945.
Request methods
HEAD: Asks for the response identical to the one that would correspond to a GET request, but without the response body. This is useful for retrieving meta-information written in response headers, without having to transport the entire content.
GET: Requests a representation of the specified resource. Requests using GET (and a few other HTTP methods) "SHOULD NOT have the significance of taking an action other than retrieval
Data retrieval
Data retrieval, in database management, involves extracting the wanted data from a database. The two primary forms of the retrieved data are reports and queries....
". The W3C has published guidance principles on this distinction, saying, "Web application
Web application
A web application is an application that is accessed over a network such as the Internet or an intranet. The term may also mean a computer software application that is coded in a browser-supported language and reliant on a common web browser to render the application executable.Web applications are...
design should be informed by the above principles, but also by the relevant limitations." See safe methods below.
POST
POST (HTTP)
In computing, POST is one of many request methods supported by the HTTP protocol used by the World Wide Web. The POST request method is used when the client needs to send data to the server as part of the request, such as when uploading a file or submitting a completed form.In contrast to the GET...
: Submits data to be processed (e.g., from an HTML form) to the identified resource. The data is included in the body of the request. This may result in the creation of a new resource or the updates of existing resources or both.
PUT: Uploads a representation of the specified resource.
DELETE: Deletes the specified resource.
TRACE: Echoes back the received request, so that a client can see what (if any) changes or additions have been made by intermediate servers.
OPTIONS: Returns the HTTP methods that the server supports for specified URL. This can be used to check the functionality of a web server by requesting '*' instead of a specific resource.
CONNECT: Converts the request connection to a transparent TCP/IP tunnel
Tunneling protocol
Computer networks use a tunneling protocol when one network protocol encapsulates a different payload protocol...
, usually to facilitate SSL
Transport Layer Security
Transport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide communication security over the Internet...
-encrypted communication (HTTPS
Https
Hypertext Transfer Protocol Secure is a combination of the Hypertext Transfer Protocol with SSL/TLS protocol to provide encrypted communication and secure identification of a network web server...
) through an unencrypted HTTP proxy.
PATCH: Is used to apply partial modifications to a resource.
HTTP servers are required to implement at least the GET and HEAD methods and, whenever possible, also the OPTIONS method.
Safe methods
Some methods (for example, HEAD, GET, OPTIONS and TRACE) are defined as safe, which means they are intended only for information retrieval and should not change the state of the server. In other words, they should not have side effectsSide effect (computer science)
In computer science, a function or expression is said to have a side effect if, in addition to returning a value, it also modifies some state or has an observable interaction with calling functions or the outside world...
, beyond relatively harmless effects such as logging
Server log
A server log is a log file automatically created and maintained by a server of activity performed by it.A typical example is a web server log which maintains a history of page requests. The W3C maintains a standard format for web server log files, but other proprietary formats exist...
, caching
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
, the serving of banner advertisements
Web banner
A web banner or banner ad is a form of advertising on the World Wide Web delivered by an ad server. This form of online advertising entails embedding an advertisement into a web page. It is intended to attract traffic to a website by linking to the website of the advertiser...
or incrementing a web counter
Web counter
A web counter or hit counter is a computer software program that indicates the number of visitors, or hits, a particular webpage has received...
. Making arbitrary GET requests without regard to the context of the application's state should therefore be considered safe.
By contrast, methods such as POST, PUT and DELETE are intended for actions that may cause side effects either on the server, or external side effects such as financial transactions or transmission of email
Email
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...
. Such methods are therefore not usually used by conforming web robots
Internet bot
Internet bots, also known as web robots, WWW robots or simply bots, are software applications that run automated tasks over the Internet. Typically, bots perform tasks that are both simple and structurally repetitive, at a much higher rate than would be possible for a human alone...
or web crawler
Web crawler
A Web crawler is a computer program that browses the World Wide Web in a methodical, automated manner or in an orderly fashion. Other terms for Web crawlers are ants, automatic indexers, bots, Web spiders, Web robots, or—especially in the FOAF community—Web scutters.This process is called Web...
s; some that do not conform tend to make requests without regard to context or consequences.
Despite the prescribed safety of GET requests, in practice their handling by the server is not technically limited in any way. Therefore, careless or deliberate programming can cause non-trivial changes on the server. This is discouraged, because it can cause problems for Web caching, search engines and other automated agents, which can make unintended changes on the server.
Furthermore, methods such as TRACE, TRACK and DEBUG are considered potentially 'unsafe' by some security professionals, because they can be used by attackers to gather information or bypass security controls during attacks. Security software tools such as Tenable Nessus and Microsoft URLScan report on the presence of these methods as being security issues.
Idempotent methods and web applications
Methods PUT and DELETE are defined to be idempotent, meaning that multiple identical requests should have the same effect as a single request. Methods GET, HEAD, OPTIONS and TRACE, being prescribed as safe, should also be idempotent, as HTTP is a stateless protocol.In contrast, the POST method is not necessarily idempotent, and therefore sending an identical POST request multiple times may further affect state or cause further side effects (such as financial transactions). In some cases this may be desirable, but in other cases this could be due to an accident, such as when a user does not realize that their action will result in sending another request, or they did not receive adequate feedback that their first request was successful. While web browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
s may show alert dialog box
Alert dialog box
An alert dialog is a colloquial term for a particular type of dialog box that occurs in a graphical user interface. It is also known as an alert box, alert window, error dialog, alert popup or plainly alert.The typical alert dialog provides information in a separate box to the user, after which...
es to warn users in some cases where reloading a page may re-submit a POST request, it is generally up to the web application to handle cases where a POST request should not be submitted more than once.
Note that whether a method is idempotent is not enforced by the protocol or web server. It is perfectly possible to write a web application in which (for example) a database insert or other non-idempotent action is triggered by a GET or other request. Ignoring this recommendation, however, may result in undesirable consequences, if a user agent
User agent
In computing, a user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system...
assumes that repeating the same request is safe when it isn't.
Status codes
In HTTP/1.0 and since, the first line of the HTTP response is called the status line and includes a numeric status code (such as "404HTTP 404
The 404 or Not Found error message is a HTTP standard response code indicating that the client was able to communicate with the server, but the server could not find what was requested. A 404 error should not be confused with "server not found" or similar errors, in which a connection to the...
") and a textual reason phrase (such as "Not Found"). The way the user agent
User agent
In computing, a user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system...
handles the response primarily depends on the code and secondarily on the response headers. Custom status codes can be used since, if the user agent encounters a code it does not recognize, it can use the first digit of the code to determine the general class of the response.
Also, the standard reason phrases are only recommendations and can be replaced with "local equivalents" at the web developer
Web developer
A web developer is a software developer or software engineer who specializes in, or is specifically engaged in, the development of World Wide Web applications, or distributed network applications that are run over HTTP from a web server to a web browser....
's discretion. If the status code indicated a problem, the user agent might display the reason phrase to the user to provide further information about the nature of the problem. The standard also allows the user agent to attempt to interpret the reason phrase, though this might be unwise since the standard explicitly specifies that status codes are machine-readable and reason phrases are human-readable.
Persistent connections
In HTTP/0.9 and 1.0, the connection is closed after a single request/response pair. In HTTP/1.1 a keep-alive-mechanism was introduced, where a connection could be reused for more than one request.Such persistent connections reduce request latency
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...
perceptibly, because the client does not need to re-negotiate the TCP connection after the first request has been sent.
Version 1.1 of the protocol made bandwidth optimization improvements to HTTP/1.0. For example, HTTP/1.1 introduced chunked transfer encoding
Chunked transfer encoding
Chunked transfer encoding is a data transfer mechanism in version 1.1 of the Hypertext Transfer Protocol in which a web server serves content in a series of chunks. It uses the Transfer-Encoding HTTP response header in place of the Content-Length header, which the protocol would otherwise require...
to allow content on persistent connections to be streamed, rather than buffered. HTTP pipelining
HTTP pipelining
HTTP pipelining is a technique in which multiple HTTP requests are sent on a single HTTP connection without waiting for the corresponding responses....
further reduces lag time, allowing clients to send multiple requests before a previous response has been received to the first one. Another improvement to the protocol was byte serving
Byte serving
Byte serving is the process of sending only a portion of an HTTP/1.1 message from a server to a client. Clients which request byte-serving might do so in cases in which a large file has been only partially delivered and a limited portion of the file is needed in a particular range. Byte Serving is...
, which is when a server transmits just the portion of a resource explicitly requested by a client.
HTTP session state
HTTP is a stateless protocol. A stateless protocol does not require the server to retain information or status about each user for the duration of multiple requests. For example, when a web server is required to customize the content of a web pageWeb page
A web page or webpage is a document or information resource that is suitable for the World Wide Web and can be accessed through a web browser and displayed on a monitor or mobile device. This information is usually in HTML or XHTML format, and may provide navigation to other web pages via hypertext...
for a user, the web application
Web application
A web application is an application that is accessed over a network such as the Internet or an intranet. The term may also mean a computer software application that is coded in a browser-supported language and reliant on a common web browser to render the application executable.Web applications are...
may have to track the user's progress from page to page. A common solution is the use of HTTP cookie
HTTP cookie
A cookie, also known as an HTTP cookie, web cookie, or browser cookie, is used for an origin website to send state information to a user's browser and for the browser to return the state information to the origin site...
s. Other methods include server side sessions, hidden variables (when the current page is a form
Form (web)
A webform on a web page allows a user to enter data that is sent to a server for processing. Webforms resemble paper or database forms because internet users fill out the forms using checkboxes, radio buttons, or text fields...
), and URL-rewriting using URI-encoded parameters, e.g., /index.php?session_id=some_unique_session_code.
Secure HTTP
There are three methods of establishing a secure HTTP connection: HTTP Secure, Secure Hypertext Transfer ProtocolSecure hypertext transfer protocol
Secure Hypertext Transfer Protocol is a little-used alternative to the HTTPS URI scheme for encrypting web communications carried over HTTP. S-HTTP is defined in RFC 2660. It was developed by Eric Rescorla and Allan M. Schiffman....
and the HTTP/1.1 Upgrade header
HTTP/1.1 Upgrade header
HTTP/1.1 introduced support for the Upgrade header field. In the exchange, the client begins by making a clear-text request, which is later upgraded to Transport Layer Security . Either the client or the server may request that the connection be upgraded...
. Browser support for the latter two is, however, nearly non-existent, so HTTP Secure is the dominant method of establishing a secure HTTP connection.
Example session
Below is a sample conversation between an HTTP client and an HTTP server running on www.example.comExample.com
Example.com, example.net, example.org, and example.edu are second-level domain names reserved for documentation purposes and examples of the use of domain names....
, port 80.
Client request
GET /index.html HTTP/1.1␍␊
Host: www.example.com␍␊
␍␊
A client request (consisting in this case of the request line and only one header) is followed by a blank line, so that the request ends with a double newline, each in the form of a carriage return
Carriage return
Carriage return, often shortened to return, refers to a control character or mechanism used to start a new line of text.Originally, the term "carriage return" referred to a mechanism or lever on a typewriter...
followed by a line feed. The "Host" header distinguishes between various DNS
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
names sharing a single IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
, allowing name-based virtual hosting
Virtual hosting
Virtual hosting is a method for hosting multiple domain names on a server using a single IP address. This allows one server to share its resources, such as memory and processor cycles, in order to use its resources more efficiently....
. While optional in HTTP/1.0, it is mandatory in HTTP/1.1.
Server response
HTTP/1.1 200 OK
Date: Mon, 23 May 2005 22:38:34 GMT
Server: Apache/1.3.3.7 (Unix) (Red-Hat/Linux)
Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT
Etag: "3f80f-1b6-3e1cb03b"
Accept-Ranges: bytes
Content-Length: 438
Connection: close
Content-Type: text/html; charset=UTF-8
A server response is followed by a blank line and text of the requested page. The ETag
HTTP ETag
An ETag, or entity tag, is part of HTTP, the protocol for the World Wide Web. It is one of several mechanisms that HTTP provides for cache validation, and which allows a client to make conditional requests. This allows caches to be more efficient, and saves bandwidth, as a web server does not...
(entity tag) header is used to determine if a cached version of the requested resource is identical to the current version of the resource on the server. Content-Type specifies the Internet media type
Internet media type
An Internet media type, originally called a MIME type after MIME and sometimes a Content-type after the name of a header in several protocols whose value is such a type, is a two-part identifier for file formats on the Internet.The identifiers were originally defined in RFC 2046 for use in email...
of the data conveyed by the http message, while Content-Length indicates its length in bytes. The HTTP/1.1 webserver publishes its ability to respond to requests for certain byte ranges of the document by setting the header Accept-Ranges: bytes. This is useful, if the client needs to have only certain portions of a resource sent by the server, which is called byte serving
Byte serving
Byte serving is the process of sending only a portion of an HTTP/1.1 message from a server to a client. Clients which request byte-serving might do so in cases in which a large file has been only partially delivered and a limited portion of the file is needed in a particular range. Byte Serving is...
. When Connection: close is sent in a header, it means that the web server
Web server
Web server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....
will close the TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
connection immediately after the transfer of this response.
Most of the header lines are optional. When Content-Length is missing the length is determined in other ways. Chunked transfer encoding uses a chunk size of 0 to mark the end of the content. Identity encoding without Content-Length reads content until the socket is closed.
A Content-Encoding like gzip can be used to compress the transmitted data.
HTTP Replacements or Enhancements
- HTTP-MPLEXHTTP-MPLEXHTTP-MPLEX is a proposed enhancement to the Hypertext Transfer Protocol .HTTP-MPLEX is presented and described in .HTTP-MPLEX and HTTP-MPLEX with extended syntax is described and examined in expanded detail in the doctoral Thesis 'Enhancing HTTP to Improve Page and Object Retrieval Time With...
- HTTP(P2P)HTTP(P2P)HTTP is a protocol based on HTTP. It is intended to improve page and object retrieval performance when web servers suffer server-side congestion. Examples of server-side congestion include the Slashdot and Flash crowd effects...
- Representational State TransferRepresentational State TransferRepresentational state transfer is a style of software architecture for distributed hypermedia systems such as the World Wide Web. The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation...
(REST) - SPDYSPDYSPDY is a networking protocol for transporting web content developed by Google and used in accessing Google web services from their browser Google Chrome. Google promotes the protocol in the open-source project Chromium to augment the Hypertext Transfer Protocol protocol, achieving higher...
- A HTTP enhancement proposed by GoogleGoogleGoogle Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...
. - Waka (protocol)Waka (protocol)Waka is an application protocol intended as "a binary, token-based replacement for HTTP." It is "designed to match the efficiency of the REST architectural style."-History:...
- A HTTP replacement proposed by Roy FieldingRoy FieldingRoy Thomas Fielding is an American computer scientist, one of the principal authors of the HTTP specification, an authority on computer network architecture and co-founder of the Apache HTTP Server project....
.
Further Information
- Basic access authentication
- Content negotiationContent negotiationContent negotiation is a mechanism defined in the HTTP specification that makes it possible to serve different versions of a document at the same URI, so that user agents can specify which version fit their capabilities the best...
- HTTP compressionHttp compressionHTTP compression is a capability that can be built into web servers and web clients to make better use of available bandwidth , and provide faster transmission speeds between both...
- HxxpHxxphxxp://, sometimes h**p:// or _ttp://, is used in URLs to obscure the fact that one is linking to a http:// website. It is generally used to avoid automatic recognition by computer programs...
Related topics and technologies
- Curl-loaderCurl-loadercurl-loader is an open-source software performance testing tool written in the C programming language.- Features :curl-loader is capable of simulating application behavior of hundreds of thousands of HTTP/HTTPS and FTP/FTPS clients, each with its own source IP-address...
- HTTP/S loading/testing open-source SW - Digest access authenticationDigest access authenticationDigest access authentication is one of the agreed upon methods a web server can use to negotiate credentials with a user's web browser. It uses encryption to send the password over the network which is safer than the Basic access authentication that sends plaintext.Technically digest...
- List of file transfer protocols
- List of HTTP headers
- List of HTTP status codes
- Web cacheWeb cacheA web cache is a mechanism for the temporary storage of web documents, such as HTML pages and images, to reduce bandwidth usage, server load, and perceived lag...
- WebDAVWebDAVWeb-based Distributed Authoring and Versioning is a set of methods based on the Hypertext Transfer Protocol that facilitates collaboration between users in editing and managing documents and files stored on World Wide Web servers...